Migration of leukocytes from blood vessels into diseased tissues is important to the initiation of normal disease-fighting inflammatory responses. But this process, known as leukocyte recruitment, is also involved in the onset and progression of debilitating and life-threatening inflammatory and autoimmune diseases. The pathology of these diseases results from the attack of the body's immune system defenses on normal tissues. Thus, blocking leukocyte recruitment to target tissues in inflammatory and autoimmune diseases would be a highly effective therapeutic intervention. The leukocyte cell classes that participate in cellular immune responses include lymphocytes, monocytes, neutrophils, eosinophils, and basophils. In many cases, lymphocytes are the leukocyte class that initiates, coordinates, and maintains chronic inflammatory responses, and thus are generally the most important class of cells to block from entering inflammatory sites. Lymphocytes attract monocytes to the site, which, collectively with lymphocytes, are responsible for much of the actual tissue damage that occurs in inflammatory disease. Infiltration of lymphocytes and/or monocytes is responsible for a wide range of chronic, autoimmune diseases, and also organ transplant rejection. These diseases include, but are not limited to, rheumatoid arthritis, atherosclerosis, psoriasis, chronic contact dermatitis, inflammatory bowel disease, multiple sclerosis, sarcoidosis, idiopathic pulmonary fibrosis, dermatomyositis, skin pemphigoid and related diseases (e.g., pemphigus vulgaris, p. foliaceous, p. erythematosis), glomerulonephritides, vasculitides, hepatitis, diabetes, allograft rejection, and graft-versus-host disease.
This migration process, by which leukocytes leave the bloodstream and accumulate at inflammatory sites, and initiate disease, takes place in at least three distinct steps which have been described as (1) rolling, (2) activation/firm adhesion, and (3) transendothelial migration (Springer T. A., Nature 1990; 346:425–433; Lawrence and Springer, Cell 1991; 65:859–873; Butcher E. C., Cell, 1991; 67:1033–1036). The second step is mediated at a molecular level by chemoattractant receptors. Chemoattractant receptors on the surface of leukocytes bind chemoattractant cytokines secreted by cells at the site of damage or infection. Receptor binding activates leukocytes, increases the adhesiveness of the adhesion molecules that mediate transendothelial migration, and promotes directed migration of the cells toward the source of the chemoattractant cytokine.
A recent discovery is the existence of a large family (>20 members) of structurally homologous chemoattractant cytokines, approximately 8 to 10 kDa in size. These molecules share the ability to stimulate directed cell migration (chemotaxis) and have been collectively called “chemokines,” a contraction of chemotactic cytokines. Each chemokine contains four cysteine residues (C) and two internal disulfide bonds. Chemokines can be grouped into two subfamilies, based on whether the two amino terminal cysteine residues are immediately adjacent (C—C family) or separated by one amino acid (C—X—C family). These differences correlate with the organization of the two subfamilies into separate gene clusters. Within each gene cluster, the chemokines typically show sequence similarities between 25% to 60%.
The chemokines of the C—X—C subfamily, such as interleukin-8 (IL-8), are produced by a wide range of cell types and act predominantly on neutrophils as mediators of acute inflammation. Chemokines of the C—C subfamily are also produced by a wide variety of cell types. These molecules act predominantly on subsets of mononuclear inflammatory cells. Currently there are at least six C—C chemokines with known chemotactic activity for human monocytes and/or T cells, including MCP-1, MCP-2, MCP-3, MIP-1α, MIP-1β, and RANTES (regulated on activation, normal T cell expressed and secreted). This suggests there may be a high degree of redundancy in chemoattractant pathways. In addition, most C—C chemokines are chemotactic for more than one cell type. For example, RANTES acts on memory CD4+ T cells, eosinophils, and monocytes. Monocyte chemoattractant protein-1 (MCP-1), another C—C chemokine, acts on monocytes, activated “memory” T cells and on basophils. MCP-1 is also a potent secretogogue of inflammatory mediators for monocytes and basophils.
Five C—C chemokine receptors have recently been characterized (CKR1–5 or CCR1–CCR5), and all of these belong to the seven transmembrane spanning G protein-coupled receptor family. Each of these receptors mediates the binding and signaling of more than one chemokine. For example, the CCR1 receptor binds both MIP-1α and RANTES. There are two receptors which bind MCP-1, namely CCR2 (with alternately spliced forms, 2A and 2B), and CCR4. CCR2 is also known to mediate binding and signaling of MCP-3. The CCR4 receptor binds and signals, in addition to MCP-1, with RANTES and MIP-1α. Which of these is responsible for the MCP-1 mediated recruitment of monocytes and T cells is not known.
In agreement with the observation that lymphocyte migrate into inflammatory sites is usually accompanied by migration of monocytes, MCP-1 is expressed at sites of antigen challenge and autoimmune disease. However, analyses of human inflammatory lesions with antibodies to other chemokines show RANTES, MIP-1α, MIP-1β, and MCP-3 to be present as well. Injection of MCP-1 into skin sites in mice provokes only a mild monocytic infiltrate or no infiltrate at all (Ernst C. A. et al., J. Immunol., 1994; 152:3541–3544). Whether these results reflect redundant and complex recruitment pathways has not been resolved. MCP-1 and MCP-3 may play a role in allergic hypersensitivity disease. This is suggested by the observation that MCP-1 lacking the amino terminal glutamic acid loses the ability to stimulate basophil mediator release and acquires activity as an eosinophil chemoattractant.
Chemokines of both subfamilies may bind to heparin sulfate proteoglycans on the endothelial cell surface, and may function principally to stimulate haptotaxis of leukocytes that attach to cytokine-activated endothelium through induced adhesion molecules. Additionally, MCP-1 has been reported to selectively activate the β1 integrin family of leukocyte adhesion molecules, suggesting a role in leukocyte interactions with the extracellular matrix. Hence, MCP-1 may not only trigger the initial arrest and adhesion of monocytes and T cells, but may also act to guide their migration in extravascular space.
Chemoattractants appear to be required for transendothelial migration in vitro and in vivo and can induce all steps required for transmigration in vivo. Injection of neutrophil chemoattractants into skin or muscle leads to robust migration of neutrophils from the vasculature and accumulation at the injection site. Pretreatment of neutrophils with pertussis toxin inhibits migration into inflammatory sites (Spangrude et al. J. Immunol. 1985; 135(6):4135–4143; Nourshargh and Williams J. Immunol. 1990; 145(8):2633–2638). Moreover, administration of a neutralizing monoclonal antibody against IL-8 markedly inhibits neutrophil migration in inflammation (Sekido et al., Nature 1993; 365(6447):654–657).
Chemoattractants impart directionality to leukocyte migration. By contrast with intradermal injection, intravascular injection of IL-8 does not lead to migration (Hechtman et al. J. Immunol. 1991; 147(3):883–892). Cytokine-stimulated endothelial monolayers grown on filters secrete IL-8 into the underlying collagen layer. Neutrophils added to the apical compartment migrate into the basilar compartment, but not when the IL-8 gradient is disrupted by addition of IL-8 to the apical compartment (Huber et al., Science 1991; 254(5028):99–102).
The endothelium may present chemoattractants to leukocytes in a functionally relevant way, as well as providing a permeability barrier that stabilizes the chemoattractant gradient. Since leukocytes, responding to specific antigen or inflammatory signals in tissue, may signal migration of further leukocytes into the site, a chemoattractant was sought in material secreted by mitogen-stimulated mononuclear cells (Carr et al. Proc. Natl. Acad. Sci. USA. 1994; 91(9):3652–3656). Purification to homogeneity guided by a transendothelial lymphocyte chemotaxis assay revealed that MCP-1, previously thought to be solely a monocyte chemoattractant, is a major lymphocyte chemoattractant. An activated subset of memory lymphocytes respond to MCP-1. In the same assay, lymphocytes respond to RANTES and MIP-1α but less so than to MCP-1 (C—C chemokines) and not at all to IL-8 or IP-10 (C—X—C chemokines). This physiologically relevant assay suggests that C—C chemokines tend to attract both monocytes and lymphocytes. In agreement with the observation that lymphocyte migration into inflammatory sites is accompanied by migration of monocytes, MCP-1 is abundantly expressed at sites of antigen challenge and autoimmune disease and, together with other chemokines, is an excellent candidate to provide the Step B signal required to activate integrin adhesiveness and migration of lymphocytes in vivo (Springer, Cell 1194; 76:301–314).